As a result of the recent technical developments for portable phones and terrestrial digital broadcasting, wireless receivers that receive data, audio, television broadcast and the like by use of wireless radio waves without wire communications have been widespread. For receiving wireless radio waves, the wireless radio waves are converted into electric signals by use of an antenna or the like, and then the signals are subjected to amplification, demodulation, signal processing and the like so as to decode the data.
Examples of antennas frequently used for portable equipment include a monopole antenna, a dipole antenna, a reversed F antenna and the like. Among them, a dipole antenna as a balanced feed antenna has figure-eight directivity characteristics, and thus if its directivity is expressed on a plane, the antenna has symmetric directivity characteristics with a feeding point 3 of the dipole antenna 1 at the center as shown in FIG. 14. The dipole antenna has directivity characteristics of rotational symmetry about the axis, and thus, when the dipole antenna is established horizontally to the ground, it exhibits the figure-eight directivity characteristics not only relative to a plane horizontal to the ground but similarly to a plane vertical to the ground.
A basic dipole antenna has the above-mentioned directivity characteristics. It is possible to enhance the directivity characteristics by forming the antenna in combination with a director and/or a reflector.
FIG. 15 shows the differences in the antenna directivity characteristics when combining a dipole antenna with a director and a reflector. As shown in FIG. 15B, in a case where a director 4 is arranged in front of the dipole antenna 1 and a reflector 5 is arranged behind, in comparison with a case as shown in FIG. 15A where the same dipole antenna 1 is used alone, the directivity is enhanced in the direction of the director 4 when viewed from the dipole antenna 1 as a radiator (i.e., a direction opposite to the reflector 5), and thus even a weak radio wave can be received.
The directivity characteristics of the dipole antenna can be changed by use of the reflector alone, and thus, more intensive directivity characteristics or wider directivity characteristics can be provided by modifying the shape of the reflector (see for example Patent Document 1).